Wave, Tide, Ocean Current, In-stream, and OTEC power

An Evaluation of the U.S. Department of Energy’s Marine and Hydrokinetic Resource Assessments. 2013. Marine & Hydrokinetic Energy Technology Committee; National Research Council.

Introduction

The U.S. Department of Energy (DOE) hired contractors to evaluate five Marine and Hydrokinetic Resources (MHK) globally: 1) Ocean tides 2) Waves 3) Ocean Currents 4) Temperature gradients in the ocean (OTEC) and 5) Free-flowing rivers and streams.

Then DOE asked the National Academy of Sciences (NAS) to evaluate the results, so NAS assembled a panel of 71 experts to write this assessment.

The NAS replied it was a waste of time for DOE to ask the contractors what the global theoretical maximum power generation from MHK resources might be. For example, solar power plants provide less than .1 % of electricity in the United States, even though the theoretical amount would be staggeringly enormous if you plastered the entire continent with them. But you can’t do that.

Nor can you fill the world’s ocean and rivers with devices to harvest the power in waves, tides, ocean currents, rivers, and temperature gradients (OTEC).

NAS says DOE should have asked was how much power could be generated locally at specific sites in the United States after taking into account technical and practical resource limits. For example:

The GIS database of MHK resources has a 100 MW resource. But after evaluating the location further, it turns out to be a 2.7 MW resource because of 1) technical resource limits (turbines 30% efficient, only 20% of the area can be used, the efficiency of connecting the extracted energy to the electric grid is 90%), and 2) practical resource issues: 50% of the remaining area interferes with existing fisheries and navigation routes, leaving a practical resource of 2.7 MW (100 MW * .30 * .20 * .90 * .50 = 2.7 MW).

Here are some more practical barriers to developing MHK:

Environmental:

Impacts on marine species and ecosystems (e.g., rare or keystone species, nursery, juvenile and spawning habitat, Fish, Invertebrates, Reptiles, Birds, Mammals, Plants and habitats)
Bottom disturbance
Altered regional water movement
acoustic, chemical, temperature, and electromagnetic changes or emissions
Physical impacts on the subsurface, the water column, and the water surface, scouring and/or sediment buildup, changes in wave or stream energy, turbulence

Regulatory obstacles:

Endangered Species Act; Coastal Zone Management Act; Marine Mammal Protection Act; Clean Water Act; Federal agency jurisdictions: National Oceanic and Atmospheric Administration (NOAA), U.S. Army Corps of Engineers (USACE), Federal Energy Regulatory Commission (FERC), State Department, U.S. Fish and Wildlife Service (FWS), Environmental Protection Agency (EPA), Bureau of Ocean Energy Management (BOEM), U.S. Coast Guard
Overlapping jurisdiction of state and federal agencies: FERC (within DOE) has jurisdiction over hydroelectric development; leases on the U.S. outer continental shelf require approval by BOEM (Dept of the Interior; NOAA (Dept of Commerce) is responsible for licensing commercial OTEC facilities; FWS (Dept of the Interior) and NOAA coordinate protection of marine mammals from potentially harmful development; NOAA also protects essential fish habitats. Projects in navigable waters fall under the jurisdiction of USACE and may also require involvement of the U.S. Coast Guard. USACE permits may be required for projects involving dredging rivers or coastal areas. The Coastal Zone Management Act involves coordination among local, state, and federal agencies to ensure that plans are in accordance with a state’s own coastal management program.

Social and economic:

Spatial conflicts (e.g., ports and harbors, marine sanctuaries, navigation, shipping lanes, dumping sites, cable areas, pipeline areas, shoreline constructions, wreck points, mooring and warping points, military operations, marine sanctuaries, wildlife refuges, Traditional hunting, fishing, and gathering; commerce and transportation; oil and gas exploration and development; sand and gravel mining; environmental and conservation activities; scientific research and exploration; security, emergency response, and military readiness; tourism and recreational activities; ocean cooling water for thermoelectric power plants that use coal, natural gas, or nuclear fuel; aquaculture; maritime heritage and archeology; offshore renewable energy; view sheds, commercial and recreational fisheries, access locations such as boat ramps, diving sites, marinas; national parks, cultural heritage sites
Interconnection to the power grid (e.g., transmission requirements, integrating variable electricity output, shore landings; Capital and life-cycle costs (e.g., engineering, installation, equipment, operation and maintenance, debris management, and device recovery and removal

TABLE 1 Issues That Impact the Development of the Practical MHK Resource
No Commercial scale MHK plants exist because:

Once installed, MHK devices are subject to mechanical wear and corrosion that is more severe than land-based equipment

Corrosion-related problems (i.e. galvanic, stress, fatigue, biocorrosion) and marine fouling are key challenges for all MHK devices. Advanced structural materials with appropriate coatings and paints still need to be identified in order to construct the robust, corrosion-resistant components for MHK energy generation.

Survivability in hurricanes, tides, storms, large waves, and so on

This is another challenging problem, especially in shallow water. Devices can be destroyed, damaged, or moved from their moorings under the actions of rough seas and breaking waves

Making MHK devices rugged enough is expensive

Rugged MHK devices may drive up the product cost due to exotic materials or increased engineering costs. The power electronics on MHK devices will be a challenge to implement and operate reliably. In shallow tidal and riverine areas, there is a great concern that debris will affect both the efficiency and durability of any installed devices.

Capital and Life-Cycle Costs

As with any energy device or power plant, there are costs such as design, installation, operation and maintenance, removal, and replacement. The largest of these costs, and potentially the greatest barrier to MHK deployments, is the capital cost. An earlier NRC committee concluded that it will take at least 10 to 25 years before the economic viability of MHK technologies for significant electricity production will be known. A 2008 report evaluating the potential for renewable electricity sources to meet California’s renewable electricity standard found that the cost of electricity from waves and currents was higher than that from most other renewable sources and had a substantially greater range of uncertainty.

The best places for MHK are often far from urban centers

In-stream power: Alaska is by far the largest resource but it’s questionable whether it would work because rivers freeze up, the scour incurred during spring ice break-up would make year-round deployment a challenge and possibly require seasonal device removal.
Tidal resource: Alaska’s Cook Inlet
OTEC: only feasible near Hawaii, Puerto Rico, U.S. Virgin Islands, Guam, Northern Mariana Islands, and American Samoa.

Scalability

These challenges affect not only installation, maintenance costs, and electricity output, but also MHK scalability from small to utility applications

Time and Regulation

The time to get all the regulatory agencies at federal, state, and local levels to agree to a project is formidable and time-consuming. Time we don’t have given that conventional world wide oil production peaked in 2005, and peak coal and natural gas are here or will be soon.

A huge part of the NAS report was criticizing the data, models, methods, calculations, and conclusions of the 5 contractors DOE hired. We are clearly not only far from building MHK devices at a commercial scale, but also knowing where to put them and if they are economically feasible, which can only be assessed at specific sites.

Most of the ocean and rivers are too far to connect to the electric grid

Areas near the grid often conflict with other environmental, social, and economic uses (see Table 1 above).

Connection to the grid is challenging due to harsh environmental conditions, unstable load flows, variable energy output, lack of electrical demand near the generation, the length of cable from a device or array to a shore terminus, potential environmental impacts from the cable, permitting issues, and the need for specialized equipment for reactive power control.

The distance required to interconnect into the electricity system is critical, as it directly impacts the economic viability of a project. Another issue is device and equipment reliability.

The electricity from these generators then must be integrated into the power system, where then intermittent variability of the resources might become important (this makes the grid less stable).

The situation could be more complicated if there are large numbers of offshore generators, because connecting a large number of devices together with no load demand along the path of the network cable could produce an unstable system.
Tidal Power

The potential of tidal power has long led to proposals of a barrage (a dam that lets water flow in and out) across the entrance of a bay that has a large range of height between low and high tides. It would generate power by releasing water trapped behind the barrage at high tide through turbines similar to a hydro-power facility. Or this could be done with in-stream turbines similar to the way that wind turbines work.

Scale: A tidal amplitude of 3.3 feet would require over 110 square miles to produce 100 MW (enough to power about 70,000 homes). This is why tidal power is limited to regions with very large tides (which tend to be in the northern latitudes, far from any cities that could use the power). Even with a current speed of 3 meters per second, a 100 MW project would need a flow of nearly 40,000 cubic meters per second, which requires 120 turbines, each having a cross-sectional area of 120 square yards, or 24 turbines of 82-foot diameter. Many more turbines would be needed for more typical, smaller currents. This many large turbines are likely to interfere with existing water uses, and an array this large would have near-field back effects that reduce the current each individual turbine experiences.

More than 1 channel: Power is reduced if there’s more than 1 channel, which also tends to divert flow to other channels.

Engineering challenges: Corrosion, biofouling, and metal fatigue in the vigorous turbulence typically associated with strong tidal flows.

Conflicting uses: Some of the locations with the highest tidal energy density are also estuaries having ports with heavy commercial shipping traffic. It is likely that there will be limitations to the number and size of turbines and the depth at which they can be deployed so as not to interfere with established shipping lanes.

Tides only generate power two to four times a day.
Wave Power

Power in ocean waves originates as wind energy transferred to the sea surface when wind blows over large areas of the ocean. The resulting wave field consists of a collection of waves at different frequencies traveling in many directions.

If energy is removed by a wave energy device from a wave field at one location, less energy will be available in the shadow of the extraction device, so a second row of wave energy devices won’t perform as well as the first row. The planning of any large-scale deployment of wave energy devices would require sophisticated, site-specific field and modeling analysis of the wave field and the devices’ interactions with the wave field.

Scale

One theoretical study on wave-device interaction modeled the Wave Dragon Energy Converter deployed in the highly energetic North Sea. They concluded that capturing 1 GW of power would require the deployment of a 124-mile-long single row of devices or a 5-row staggered grid about 1.9 miles wide and 93 miles long. This doesn’t take into account that the recovered power must be transformed into electricity and then transmitted. Because of the high development and maintenance costs, low efficiency, and large footprint, such devices would be a sustainable option only for small-scale developments considerably less than 1 GW close to territories with limited demand, such as islands.

It would take about 81 miles of wave machines to produce as much power as a typical power plant (1000 MW). Even if you built wind machines as far north as Canada and as far south as Mexico along both coasts, you’d only get 9% of the electricity we use now (Hayden).

Wave Power Efficiency

None of these systems are likely to operate at efficiencies over 90% and will probably have more realistic efficiencies of 50-70%. This calls into question claims of wave energy facilities that capture 90% or more of the available energy.

Other Wave Power Issues

Waves are intermittent, which means energy production is spotty
Waves have a low potential energy that varies with the weather and only a small hydraulic head of 2 or 3 meters. Hence large volumes of water have to be processed which means large structures relative to power output
The waves are a challenge for energy harvesting since they not only roll past a device but bob up and down or converge from all sides in confused seas, plus have to cope with the period of the wave (Levitan)
no design that’s been investigated is very good at capturing a very large fraction of the energy over a range of wave conditions. If they’re designed to efficiently capture wave energy in “average” sea conditions, they’ll be totally overwhelmed in high sea conditions. If they’re designed for efficient energy capture in high sea conditions, they’ll be almost totally insensitive to the energy present in average conditions (HED).
These devices typically produce what’s known as low-frequency power, which can be difficult and expensive to convert to high-frequency electrical grids
Wave technologies have lots of electrical components, hydraulic fluids and oils — all presenting a pollution risk
So far about 30 wave power ventures have failed, such as Denmarks “Wave Dragon”, the UK “Salter Duck”, Netherlands “Archimedes Wave Swing”, The Sea Clam, the Tapchan, the Pendulor.

Ocean Current Power

Ocean currents (excluding tidal currents) are affected by Coriolis forces and mainly generated by winds that cause strong, narrow currents which carry warm water from the tropics toward the poles, such as the Gulf Stream, with an ocean current in the Florida Strait that can exceed two meters per second.

The ocean current power team estimated the Florida current could generate 14.1 GW, or 62% of the 20 GW maximum power obtainable.

NAS thought that figure was way too high for many reasons and concluded that maximum power that could be extracted is 1 and 2 GW at best.

Or it may be less than 1-2 GW:

If the high turbine density in the water column diverted the Florida Current and forced the flow around the Bahamas
Seasonal variability and meandering might limit the placement of turbines to just a few narrow areas where the flow was consistent

Ocean Thermal Energy Conversion (OTEC) Power

Ocean thermal energy conversion (OTEC) is the process of deriving energy from the difference in temperature between surface and deep waters in the tropical oceans. The OTEC process absorbs thermal energy from warm surface seawater found throughout the tropical oceans and ejects a slightly smaller amount of thermal energy into cold seawater pumped from water depths of approximately 1,000 meters. In the process, energy is recovered as an auxiliary fluid expands through a turbine.

NAS thought the study should have been limited to just the areas this could possibly work: the Hawaiian Islands, Puerto Rico, U.S. Virgin Islands, Guam, the Northern Mariana Islands, and American Samoa. Hawaii could generate 143 TWh/yr, the Mariana Islands (including Guam) 137 TWh/yr, and Puerto Rico and the U.S. Virgin Islands 39 TWh/yr. The majority of this resource is found far from the United States near Micronesia (1,134 TWh/yr) and Samoa (1,331 TWh/yr).

The continental U.S. resource is very seasonal and limited, and it is unlikely that plant owners would want to operate only part of the year.

OTEC plants are vulnerable to corrosion, strong currents, tides, large waves, hurricanes, and storms, and remaining anchored.

OTEC could cause environmental damage.

OTEC plants must be near tropical islands with steep topography to make it easier to reach deep cold water and transmit power to shore.

The committee estimated the global OTEC resource could be 5 TW (a 100-MW plant every 30 miles in the tropical ocean). In reality, this would never happen because you need to connect them to land-based electric grids.

OTEC needs very large equipment and very high seawater flow rates

OTEC systems are similar to most other heat engines. There are significant practical aspects that make it difficult to implement, mainly from the small available temperature difference of only ~20ºC between the warm and cold seawater streams. Because of the low efficiencies, OTEC plants require very large equipment (e.g., heat exchangers, pipes) and seawater flow rates (~200-300 cubic meters per second for a typical 100-MW design) that exceeds any existing industrial process to generate a significant amount of electricity.

OTEC needs to be near existing electric power systems

The cold-water pipe is one of the largest expenses in an OTEC plant. As a result, the most economical OTEC power plants are likely to be open-ocean designs with short vertical cold-water pipes, close enough to shore to connect to existing electric power systems.

Concerns with tides, variation in power output, shear current effects on the cold-water pipe

The committee is concerned about the variations in isotherm depth due to internal tides, which can be significant near islands. For example, deep isotherm displacements of as much as 50 or even 100 m are common near the Hawaiian Islands, which could induce a 5-10 percent variation in power output over the tidal cycle. In addition, areas with strong internal tides will also impose strong shear currents on the cold-water pipe. Seasonal variations could lead to a 20% variation in power output in Hawaii over the course of the year. Even more dramatic changes result from fluctuations due to El Niño or La Niña in the central tropical Pacific, where the committee estimates variations in power production as high as 50 percent. The assessment group largely fails to address the temporal variability issue.

Spacing must be far apart given the huge seawater requirements

Clearly, a key question for determining the OTEC technical resource would be how closely plants could be spaced without interfering with each other or excessively disturbing the ocean thermal structure. At regional and global scales there could be a variety of impacts on the ocean arising from widespread deployment of OTEC.

There are many interesting physics, chemistry, and biology problems associated with the operation of an OTEC plant. Whitehead suggested that an optimal plant size would be around 100 MW in order to avoid adverse effects on the thermal structure the plant is designed to exploit.
In-Stream Hydrokinetic Power

In-stream hydrokinetic energy is recovered by deploying a single turbine unit or an array of units in a free-flowing stream. Estimates of the maximum extractable energy that minimizes environmental impact range from 10 to 20% of the naturally available physical energy flux.

There are many limiting factors that will reduce the in-stream hydrokinetic energy production

These factors include but are not limited to ice flows and freeze-up conditions, transmission issues, debris flows, potential impacts to aquatic species (electromagnetic stimuli, habitat, movement and entrainment issues), potential impact to sites with endangered species, suspended and bedload sediment transport, lateral stream migration, hydrodynamic loading during high flow events, navigation, recreation, wild and scenic designations, state and national parklands, and protected archeological sites. These considerations will need to be addressed to further estimate the practical resource that may be available.

Navigable waters are a resource for a number of sectors, and coordinating their use is an immense logistical challenge that will definitely impact in-stream energy development.

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